1. Technical Field
The present invention relates to a technique of adjusting ink ejection of nozzles of an inkjet head. Specifically, the invention relates to the technique of suppressing unevenness of ejection amount of ink due to uneven characteristics of ink ejection among a plurality of nozzles of the inkjet head.
2. Prior Art
Conventionally, in a field of an inkjet printer, it has been known that the unevenness of the thickness is observed on an image formed on a print sheet due to unevenness of the ink ejection characteristics among the nozzles of the inkjet head.
The unevenness of the ink ejection characteristics may include unevenness of resistance in ink flow paths connected to the nozzles due to respective shapes thereof, unevenness of characteristics of actuators applying ink ejection energies to the ink and the like. Because of such causes, among a plurality of nozzles, the ink ejection characteristics, for example, ink ejection amount from each nozzle, a speed of the ink ejected from each nozzle, are varied. When the ink ejection characteristics are varied among the plurality of nozzles, quality of the image is deteriorated since the thickness of the formed image appears to have unevenness as the size of the dots formed on the print sheet may be uneven, positions of the dots are displaced. In order to cope with such a problems, there has been know a technique to suppress the unevenness of the thickness of the image due to the unevenness of the ink ejection characteristics by adjusting an ink ejection condition for each of the plurality of nozzles.
There is known a technique of printing an image pattern to obtain information of the unevenness of image thickness, which information is necessary for compensating the unevenness of the ink ejection characteristics among the plurality of nozzles. Specifically, according to the above patent, four filled patters, which are the unevenness detection patters, are formed using the nozzles aligned in a line. Next, the four filled patterns are scanned using an image sensor, and obtain thicknesses of the portions formed by the respective nozzles. Then, based on thus obtained thickness data for each of the nozzles, a thickness compensation is performed for each nozzle when an image is formed on the print sheet. According to this technique, four pieces of thickness data corresponding to the four patterns are obtained, which may be averaged or the mode value may be used.
Incidentally, it is preferable that a gap between the inkjet head and the print head is a fixed and ideal one over an entire area of the print sheet. However, in practice, the print sheet may include rising portions, warpage portions and corrugated portions, and the gap may not be constant over the entire area of the print sheet.
Under such a condition, if the thickness measuring patters as disclosed in the above-described patent are formed, the thickness of the patters may be changed due to the unevenness of the gap. An example of such a problem will be described below.